1988 Mercury 115 with low compression

[SFCNay]

OK here we go. I purchased a 1988 Basstracker about a year ago. It started when I bought it but was never lake tested. When I got it home my brother in law did a compression test on it and it had 2 low cylinders. The boat sat all winter and this spring we took it to someone who advertised a rebuilt powerhead online. He rebuilt the powerhead that was in the motor and we lake tested it with him when we picked it up. Now here comes the problems. The boat sat for almost a year with gas in the tank. I have since learned that aint good. Instead of emptying the tank he just added more gas to it so that it would have some good gas in it. When we went to pick it up it would not start easily and he had to use starter fluid to get it going. When we went on the lake with it it would only do about 4700 or so RPM at WOT. He told me that I would need a new prop. It sat for about another month before I could take it out again and when I tried to start it in the yard it wouldnt start. Drained the carbs and lines and they were full of water. Completely drained the tank of about 15 gallons of gas and oil (dont know what to do to dispose of that either) and put new gas and oil and it started. Took it out to the lake without the new prop and same thing. Right about 4700 RPM max. Then it dropped to 3000 RPM max with huge loss of power. Took it back in and checked the plugs and the 2 center plugs had what looked like tin foil on them. One was much worse and actually had it built up around the plug inside the piston. We broke that loose and pulled as much as we could out and turned it over a couple times to try to blow the metal out of the cylinder. Put the plugs back in and went back out. Started right up, gobs and gobs of power 5200 or so RPM at WOT for about 4 min then back down to 4700. Then 3000 on the way back to shore. Checked the plugs again and more metal but not nearly as much. Cleaned the plugs and off again. 5200 RPM plenty of power for alot longer then 4700 again. Checked plugs again and only 1 was fouled the other was fine. Cleaned it up and off again. Didnt have any more problems the rest of the day. Starts fine when it is warm. Took it back out again last weekend and it is a real pain to get started. Takes forever but once I got it started it seems to run ok. Starts right back up after it is warm. Checked the plugs several times while I was out there and they never fouled. When I got home I bought a compression tester and checked the compression on it cold. All of the cylinders were rigth around 110# except the 2 that fouled the plugs. one is at 100# and the one that had metal built up in the cylinder is just over 90#. Now my question is.......What caused the problem and what can I do to fix it and can I still use this motor or am I going to ruin it running it with the low compression.

 

[CharlieB]

Re: 1988 Mercury 115 with low compression

I hate to be the bearer of bad tidings, but............

The two cyl's that fouled the plugs with 'silvery metal' were running too lean (HOT) and melting the pistons. They are both hurt and will be needing replacement. When, is the question. Ideally, all cyl's compression readings should fall within a tight range with less than a 10% difference between the highest and lowest in order to achieve maximum performance from the motor.


Your immediate issue is the carb's are not squeeky clean, either a mineral accumulation restricting a jet, water contaminating, something caused those center cyl's to run lean enough to melt. Poor fuel quality may have contributed. possibly timing just a degree too advanced. Everything must be checked and rechecked again. You are actually lucky that you didn't solidly stick a piston and send a rod out the side of the block.

 

[Chris1956]

Re: 1988 Mercury 115 with low compression

SFC Nay, I agree with Charlie. You did get lucky, so far. The carbs need to be cleaned, and the timing and idle mixture set properly. I would also replace the waterpump impeller, if you haven't already.

Are the two low cylinders #1 and #3?

 

[85Lund]

Re: 1988 Mercury 115 with low compression

I had the same issue on my '85 155 and my timing was not correct. I got lucky and my guy set it after the first time this happened to me. I replaced all plugs after the timing was set and have not had the problem since. He did tell me that if it kept happening I would need to have the pistons replaced. With the timing turned back I think I lose 1 or 2 mph and a little power, but for me that is fine because I don't run her wide open anyway.

 

[j_martin]

Re: 1988 Mercury 115 with low compression

The original problem that burned down a coupla cylinders wasn't taken care of. Anybody that would "rebuild" a power head and not check out the carburetion, fuel system, and ignition system thoroughly is not a mechanic, he's a parts changer.

What happens is dirty fuel plugs up a carb, and cylinders run lean and burn down. Fixing the melted pistons without fixing the carb and doing something about the dirty fuel is just plain foolish.

Go at it again, only get serious this time.

hope it helps
John

 

[SFCNay]

Re: 1988 Mercury 115 with low compression

So if it hasnt fouled again do you think that the fuel problem is fixed? I know that it had some extremely old fuel in it when I dropped it off with him and I completely drained the tank when it would not start for me in the yard prior to me taking it out to the lake the first time. When I finally checked the carbs they were full of water. So I drained the carbs and lines and tank. Could the fouling that I had been leftover from the bad gas? It fouled less and less every time I checked it and now hasnt fouled again. The guy that rebuilt it for me gave me a 90 day warranty if he was the one to install it which he was. My concern is that I dont want to have to have the same problem again. That is if he even honors my warranty. Never know when you take it to someone who works out of his own shop

 

[Clams Canino]

Re: 1988 Mercury 115 with low compression

First of all it's not just "fouling", and while the water remaining was a sure nusience, it isn't / wasn't your problemnow. The deflector domes on the pistons got overheated due to pre-ignition / detonation and began to sling off aluminum. The aluminum fouled the plugs - shutting down the cylinders and "saving" the pistons from further damage. At that point the last thing you needed to do was "un-foul" the plugs and run it at WOT for FOUR FREAKIN' MINUTES STRAIGHTwhen you had yet to determine (or even comprehend) what was happening to you.

I think you'll find that the guy that overhauled it warned you repeatedly during the lake test that the motor felt and tached "overpropped" and to run it that way was a severe detonation risk. You were advised ot get a lesser pitch prop BEFORE running it. Then.. when detonation started to happen..... you still didn't recognize it and went on to hammer it some more - at least four minutes more at WOT. All this after being warned over and over about running these inlines at extended WOT at all - let alone overpropped and during the first break-in tank.

Eventually those 2 pistons should get attention... HOWEVER... **IF** the damage was confined to the tops of the deflectors and didn't stick the rings then running it should not hurt it more. Do a compression test after some use to make sure it does not drop more in those two. You still didn't answer what 2 it was?

-W (longer post to follow)

 

[Clams Canino]

Re: 1988 Mercury 115 with low compression

As you might gather, not just "any" guy with a shop rebuilt this powerhead - I did. I can assure you that the timing and sync were correct. In fact I follwed the very FAQ here on I-boats. No wait!... In fact I'm the **author** of that very FAQ article here on Iboats.

Those of you that know me.. know better. And if you don't, go look at the FAQ for setting up Merc inlines. I'm not just "some guy with a shop".

Not only did I set the timing but I cleaned the carbs and replaced the fuel pump. As is my SOP - I disabled the "economizer" circuit in the carbs to avoid a lean-out. I also warn people about running at extended WOT unless they are SURE everything in the fuel system is at 110% perfection. Even a small vacuum leak at the fuel connector can "show up" to hose you at WOT. My standard line is... Before you go WOT, you gotta ask yourself one question... Do you feel lucky?......... Well.... Do ya?

As for the gas - it was about a year old and very well stabilized. It smelled new and after 30 years sniffing gas (not glue) - I know the difference. I siphoned a couple gallons off the bottom to try to get anything that might be there, and then added about twice as much new gas as to what remained. And double oil for break in. I also advised the client to add a bottle of "dry gas" to emulsify any remaining water. None the less, water can cause skipping, missing, HARD STARTING, but not what we see here.

Direct Charge Inline piston domes (deflectors) melt from 3 things; together, or in combination that can (and will) cause detonation.

1.Too much spark advance
2. Poor octane gas (not enough antiknock compounds causing it to "flash" early.)
3. Overloading the motor (overpropping)

Since we know this motor was timed correctly and had clean carbs. We need to pay close attention to the fact that I told him during the water test that it was over-propped. Not once did I warn about this... but at least 4 times.
I would not run it WOT for more than 10 seconds to check the tach... and warned him over and over not to do so until he got a different prop on there. I could not have been plainer.

He was **supposed** to run this double oil tank of mixed gas (after adding emulsifier) through nice and easy to break it in is all. The fact that there was a water pocket still trapped in the bottom is only a nusiance here, and a distraction from the meltdown. water in the gas never elevated piston temps. Water will cause missing, stumbling, **hard starting** etc etc. But it doesn't melt pistons.

The boat was IMHO severely overpropped. I think it's obvious by the fact that after suffering a "near melt-down" he cleared the aluminum and hammered it AGAIN for four minutes - that he lacked a good understaning of the forces in play against his motor ; this despite my repeated warnings about propping and about the difference between normal operating RPM's and WOT operation. (operating is 4000-4200)

And I'm sorry the whole thing got posted to here.. BEFORE I ever even got an email that there was a problem at all.

-W